An ion guide (or ion transport apparatus) may be utilized to transmit ions in various types of ion processing devices, one example being a mass spectrometer (MS). The theory, design and operation of various types of mass spectrometers are well-known to persons skilled in the art and thus need not be detailed in the present disclosure. A commonly employed ion guide is based on a multipole electrode structure in which two or more pairs of electrodes are elongated in the direction of the intended ion path and surround an interior space in which the ions travel. Typically, the electrode structure is an RF-only electrode structure in which the ions passing through the ion guide are subjected to a two-dimensional, radio-frequency (RF) trapping field that focuses the ions along an axial path through the electrode structure. The paths of the ions are able to oscillate in radial directions in the transverse plane that is orthogonal to the axis of the electrode structure, but these oscillations are limited by the forces imparted by the RF electrical field being applied in the transverse plane. As a result, the ions are confined to an ion beam centered around the axis of the electrode structure (which typically is a geometrically centered axis). In the absence of the RF field, the ions would be widely dispersed in an unstable, uncontrolled manner. Few ions would actually be transmitted to a subsequent device from the ion exit of the ion guide; most ions would not reach the ion exit but instead hit the ion guide rods or escape from the electrode structure. Therefore, in an ion guide the ions need to experience a certain minimum amount of RF restoring force during their flight so as to be confined to an ion beam for efficient transmission to and beyond the ion exit at the axial end of the ion guide.
In a conventional ion guide, the applied RF electrical field is generally uniform along the axial direction from the ion entrance to the ion exit, disregarding fringe effects and other localized discontinuities. As a result, the ion beam is generally cylindrical at least in the sense that the cross-sectional area of the ion beam—generally representing the envelope in which radial excursions of the ions are limited in the two-dimensional plane—is uniform along the axis. The size of the cross-section of the ion beam generally depends on the nature of the RF field being applied. As examples, a set of four parallel electrodes may be utilized to generate a quadrupolar RF field, a set of six parallel electrodes may be utilized to generate a hexapolar RF field, etc. In a quadrupolar field, the ions are focused more strongly about the axis and hence the cross-section of the ion beam is smaller as compared to a hexapolar field. In all such conventional cases the RF field and therefore the cross-section of the ion beam are uniform. However, the conditions under which ions of a given mass-to-charge (m/z) ratio or range of m/z ratios can be admitted into the ion guide in an optimal manner are not necessarily the same as the conditions under which ions can be emitted from the ion guide in an optimal manner. Consequently, the dimensions of a uniform ion beam are often not optimal for both ion entry and ion exit, or even for either ion entry or ion exit alone, leading to less than optimal ion signal and instrument sensitivity.
Accordingly, there is a need for ion transport devices configured for providing optimized ion transmission conditions for ions of a wide range of m/z ratios.